1. Field of the Invention
The present invention relates to a cellular wireless communication system and base station, and is suitably applied to, for example, a portable telephone system.
2. Description of the Related Art
Heretofore, in a cellular wireless communication system, an area for offering the communication service is divided into cells of the desired size and base stations as the fixed stations are provided in each cell and a communication terminal device as a mobile station to wireless communicate with the base station which is considered having the optimal communication condition. In this case, although various systems are proposed as the communication system between the communication terminal device and the base station, there is a time division multiple access system called Time Division Multiple Access (TDMA) as a typical system.
This TDMA system is a system that divides one frequency channel into time slots and transmits reception signal at the timing of slot assigned to own station, and the same frequency channel can be used in multiple communications (multiplex communications) effectively utilizing frequency.
At this point, the cellular wireless communication system of the TDMA system is described with reference to FIG. 1 as follows. As shown in FIG. 1, in the cellular wireless communication system 1, a service area is divided into multiple cells C1-C7 and base stations BS1-BS7 are provided in each of these multiple cells C1-C7. Users can wireless communicate with the base station from any position in the service area. In this case, the base stations BS1-BS7 provided in each cell C1-C7 are connected to a connection switching station 2 of the higher stage, which controls these base stations BS1-BS7, signal to be transmitted from each base station BS1-BS7, and the signal path of signals to be sent to each base station BS1-BS7 by circuit switching.
In case where the communication terminal device MS1 as a mobile station communicates in the service area of this cellular wireless communication system 1, said communication terminal device MS1 selects a base station having the best communication condition from among these base stations BS1-BS7, and can communicate user information such as audio signal by connecting with said base station via wireless circuit. For example, if the communication terminal device MS1 is located in the vicinity of the base station BS1, the optimal communication condition which can be obtained between said base station BS1 and the communication terminal device MS1, is wireless connected to said base station BS1. At this point, the base station BS1 transmits the reception signal received from the communication terminal device MS1 to the connection switching station 2 of the higher stage. Contrary to this, the base station BS1 transmits the transmission signal received from the connection switching station 2 to the communication terminal device MS1.
The connection switching station 2 applies the decoding processing, that is channel decoding, to the reception signal received from the base station BS1, and transmits the resulting reception data to a terminal device 4 of the communicating party via a public circuit net 3. Moreover, the connection switching station 2 receives data to be transmitted from the terminal device 4 of the communicating party via the public circuit net 3, forms transmission signal by applying the coding processing, that is channel encoding, and sends said transmission signal to the base station BS1. With this series of processings, the communication terminal device MS1 can communicate with the terminal device 4 of the communicating party via the base station BS1, the connection switching station 2 and the public circuit net 3.
However, as the communication terminal device MS1 moves from one place to another, there are cases where the base station to which said communication terminal device MS1 to be connected is changed. For example, this example applies to the case where the communication terminal device MS1 moves from a cell C1 that is the service area of the base station BS1, to a cell C2 that is the service area of the base station BS2. At this point, the communication terminal device MS1 conducts the connection switching of wireless circuit from the base station BS1 to the base station BS2, i.e., handoff, and conducts wireless communication with newly connected base station BS2. By conducting such handoff, even in the case where the communication terminal device MS1 moves, communications can be continued by sequentially switching base stations.
In order to conduct the handoff, it is necessary to constantly look for "the mobile station should be connected to which base station". In general, in the cellular wireless communication system 1, the communication terminal device MS1 which is a mobile station, receiving the signals of the control channel (hereinafter referred to as control channel CCH) to be transmitted from each base station BS1-BS7, measures these reception power respectively, and determines the base station having the largest reception power as a candidate for the base station of handoff destination. Thus, the mobile station side determines a base station candidate of the handoff destination.
In this connection, the final decision on whether to conduct the handoff or not is made by the base station side. As the flow of handoff processing, "the mobile station finds a candidate for the handoff destination, and based on this result, the base station conducts the handoff". More specifically, the base station constantly monitors whether the reception power from the communication terminal device in communication is enough or not enough, and in case where satisfactory quality of communication cannot be obtained due to shortage of the reception power, the base station decides the handoff of said communication terminal device to another base station. In that case, the base station executes handoff to the base station which is notified from the communication terminal device as a candidate for handoff. In this connection, there are cases where the communication terminal device demands the handoff due to drop of the reception power in the communication terminal device from the base station. However, also in this case, the final decision whether to conduct the handoff or not is made by the base station side.
In the conventional cellular wireless communication system 1, there are cases where the base station is changed successively as the communication terminal device moves, however, it basically communicates with one base station. In this case, in the cellular wireless communication system 1, one base station which is considered having satisfactory communication condition is selected relative to the reception power, and makes said base station and the communication terminal device communicate each other. However, in practice, the base station having the satisfactory communication condition is not limited to one station but there are many cases where multiple base stations having favorable communication conditions exist. In that case, not selecting one base station to communicate but rather using the reception signals received by such base stations with satisfactory communication conditions, it is considered that the quality of upward communication (signal direction from the communication terminal device to the base station) can be easily improved.
Furthermore, in such a conventional cellular wireless communication system 1, the reception power of the downward control channel CCH to be transmitted from the base station is measured in the communication terminal device, a base station candidate of handoff destination is found based on said measured power, and said candidate is notified to said base station. And in the base station, when the reception power of the upward signal to be transmitted from the communication terminal device becomes no longer enough, making said timing as a trigger, said communication terminal device is conducted handoff to the base station of said handoff destination. However, if the handoff is conducted according to this method, it causes a problem that the handoff is conducted to the most satisfactory base station from the standpoint of communication terminal device but said base station is not necessarily the best base station in the whole system. More specifically, in the handoff method of the conventional cellular wireless communication system 1, the destination of handoff is decided by the decision of one side, and thus caused an inconvenience as described above. Accordingly, if the destination of handoff is determined considering all aspects, it is considered that the optimal base station can be found, and the quality of communication between the communication terminal device and the base station can be improved.